Magnetoresistive (MR) sensors typically include a pair of ferromagnetic layers separated by a thin nonferromagnetic “spacer” layer. One of the ferromagnetic layers usually has a magnetic moment that is fixed or “pinned” in direction despite provision of an external magnetic field that causes the magnetic moment of the other “free” ferromagnetic layer to rotate. The resistance to current flow through the nonmagnetic layer varies depending upon the relative orientation of the magnetic moments, and so the rotation of the free layer relative to the pinned layer can be used to sense the external field.
The above described sensor layers are typically formed between a pair of parallel, soft magnetic “shield” layers that extend in a track-width much further than the free layer. The shields intercept magnetic fields that are directed at the free layer from bits of a magnetized medium track that are not directly opposite to the free layer, allowing the closest bits of the track to be more easily sensed by the free layer.
Commercially available spin-valve sensors operate with electric current flowing along the plane of the spacer layer, and may be termed current-in-plane (CIP) sensors. Alternatively, current-perpendicular-to-plane (CPP) sensors are designed to have electric current flowing perpendicular to the plane of the spacer layer, typically between the pair of shields, so that the shields also serve as leads for the sensor. To increase storage densities, CPP sensors have been proposed with shields that are also located on each side of the free layer, so that the free layer does not sense signals from adjacent tracks.
An antiferromagnetic (AF) layer may be used to set the magnetization of the pinned layer. The free layer may have multiple magnetic domains especially near its edges, which can cause noise and reduce the magnetoresistive effect. For that reason, a biasing mechanism may be provided for the free layer that reduces edge effects. CIP sensors are typically biased by hard magnetic layers that are located next to the edges of the free layer. Such longitudinal biasing would interfere with side shields, and so a CPP sensor has been proposed that has an in-stack longitudinal bias layer that is pinned by a second AF layer. Setting the magnetization of the bias layer, however, may destabilize the magnetization of the pinned layer.